The effect of estrogen, progesterone and thyroxine on basal and thyrotropin induced prolactin release in vivo and in vitro will be investigated in cattle. In addition, changes in bovine pituitary prolactin availability with changes in physiological states will be investigated by administering a standard thyrotropin releasing hormone (TRH) challenge to cows on day 0, 1, 4, 11, 18 and 20 of the estrous cycle and at 2, 4, 6, 8 and 10 months of lactation and measuring changes in serum prolactin. Blood will be collected from a pudic artery and mammary vein immediately before and after milking or TRH induced prolactin release and serum prolactin will be quantitated to determine arteriovenous differences across the mammary gland. Incorporation of H3-leucine into prolactin by bovine pituitary pieces in vitro will be quantified by immunoprecipitation. The partition of label between stored and released prolactin will be evaluated and the effect of TRH on prolactin synthesis will be determined.